The present invention relates to a cell driving type piezoelectric/electrostrictive actuator. More particularly, it relates to a cell driving type actuator having cells each being formed independently by two piezoelectric/electrostrictive elements, wherein the piezoelectric/electrostrictive elements forming the cells are displaced preferably by a driving electric field applied in the same direction as the polarization field of the piezoelectric elements or in the direction perpendicular to cell wall surfaces of the electrostrictive elements.
There is known, as a conventional piezoelectric actuator, for instance, a piezoelectric actuator being driven in the shear mode and being used in an ink jet head. With reference to FIG. 7, a piezoelectric actuator 71 includes a plurality of piezoelectric elements having driving parts 74 which include comb teeth 76 on a base plate 72. Slits 75 exists between the comb teeth 76 and are closed by a cover plate 77 so as to form cells 73 in a generally rectangular parallelepiped form. An ink head 70 where the cell 73 is used as an ink chamber is constituted by closing the respective openings at the front end of comb teeth in the piezoelectric actuator 71 with a nozzle plate 9 having nozzles 8. The comb teeth 76 are deformed so as to change the volume of the cell 73 by applying a driving electric field in the direction perpendicular to the direction of the polarization field in comb teeth 76 used in driving parts 74, which are made of a piezoelectric material. Accordingly, the ink filled into the cell 73 can be ejected.
The above-discussed driving method is the shear mode method in which the driving electric field is applied in the direction perpendicular to the polarization field of the piezoelectric elements to displace the piezoelectric elements.
Such a piezoelectric actuator 71 has been constructed by the procedure shown in FIG. 8(a)-FIG. 8(e). First, a piezoelectric material 86 is provided as in FIG. 8(a), and fired as shown in FIG. 8(b). Subsequently, the polarizing treatment is carried out as shown in FIG. 8(c). In FIG. 8(d), the process of forming fine slits is carried out by using a dicing saw or the like, and the driving parts 74, which cause the dislocation due to the application of the driving electric field, are formed like teeth of a comb in alignment by interposing a plurality of slits 75 to be a space for storing the ink therein. Electrodes 88 are formed on the walls of the slits 75 as shown in FIG. 8(e). Thereafter, as shown in FIG. 7, the cells 73 to be filled with an ink are formed by applying the cover plate 77 formed by a glass plate or the like thereto, and by closing the front ends of the comb teeth 76 with a nozzle plate 9 having nozzles 8.
Such a manufacturing method has, however, the following problems due to the machining of hard piezoelectric materials.
The first problem is that it is time-consuming to machine the slits constituting the cells, so that the method is unsuitable for mass production.
The second problem is that since the resultant slits are polluted with either free abrasive grains used for machining or a liquid used for machining, a satisfactory cleaning is required after the machining process. The cleaning step is a complex process and the mechanical strength is reduced after the slits are formed. Moreover, it requires inevitably a drying process. Additionally, the cost increases since facilities and the management for cleaning water and exhaust water are required.
The third problem is that it is difficult to form cells having a high aspect ratio of, for example, more than 10 because the slits constituting the cells to be filled with an ink can not be machined with a width of approximately 60 μm or less due to the restriction derived from the thickness of the dicing blade used for the machining, and regarding the thickness of a comb tooth, i.e., a driving part, a limited value is automatically determined with respect to the depth since the grinding strength is required for the dicing blade. As a result, it is difficult to form actuator having a high density or having a high strength and a high power.
Incidentally, the aspect ratio is generally defined by the ratio of the diameter and the axial length in the case of a cylindrical aperture, whereas, in the case of non-cylindrical aperture, for instance, the slit 75 providing a cell made by closing it in subsequent processing, as shown in FIG. 8(d), the aspect ratio is defined by the ratio of the minimum spacing between two facing comb teeth 76, i.e., the width of the slit 75, and the depth of the slit 75. A cell of a high aspect ratio means a cell having a slit whose depth is relatively large in comparison with the width.
The fourth problem is that the process of bonding parts is always required in the subsequent processing when cells having a complex form are produced because only straight and flat slits can be formed due to the machining with a dicing blade. Moreover, the deformation due to the piezoelectric stress extends up to the bounded end of the nozzle plate during operation as a consequence of the straight line machining, and thus it is liable to result in the reduction in the durability of the bonding face.
The fifth problem is that the characteristics of the cells are liable to be deteriorated because side faces of comb-like driving parts 74 are apt to become uneven since the slits are formed by the grinding process after firing. FIG. 9(a) and FIG. 9(b) are drawings illustrating this effect. FIG. 9(a) is a side view of the end face viewed from Q in FIG. 8(d), and FIG. 9(b) shows an enlarged section of the part N in FIG. 9(a). In the grinding process with the dicing saw, micro cracks and transgranular fractures are often present in the side faces of comb-like driving parts 74 (comb teeth 76) due to the machining. As a result, it sometimes happens that the intrinsic performance of the material is not attained or that the device itself breaks due to the propagation of micro cracks when the cells are driven.
Moreover, in the conventional piezoelectric actuator 71, there are several problems attributed to the operation in the shear mode.
The sixth problem subsequent to the fifth problem is that, after firing and carrying out the polarization treatment, the manufacturing process including heating at a temperature higher than the Curie temperature cannot be carried out because the polarization in the piezoelectric material melts away. Therefore, in the case of fixing or wiring the actuator to, e.g., a circuit board, neither soldering by a reflow soldering method or the like nor bonding while heating can be carried out, otherwise, such a process suffers a thermal restriction, and thus throughput is reduced, thereby increasing the cost of manufacturing. Moreover, a machining process inducing heat, such as laser processing or the like, also provides such a restriction.
In addition, as the seventh problem, it can be pointed out that the actuator cannot be driven with a high field strength which provides a change in the state of polarization since the driving electric field is applied in the direction perpendicular to that of the polarization field. The high driving field strength gradually changes the state of polarization during the period of operation, hence reducing the magnitude of the strain. As a result, the basic performance of the actuator is reduced.
Moreover, in the conventional piezoelectric actuator 71, there is a problem due to the structure in which the base plate, driving parts, and the cover plate are integrated in one body, inclusive of the problems resulting from the aforementioned method for machining and the problem inherent in the shear-mode operation.
The eighth problem is that it is impossible to make adjacent cells behave in the same way. FIG. 15 shows sectional views of an embodiment of piezoelectric actuator 71 in deactivated and activated states. In the case that the driving electric field is in OFF state, the driving parts 74, i.e., the piezoelectric elements are not deformed, whereas in the case that the driving electric field is ON state to the specified driving parts, the driving parts 74 are deformed. As obvious from FIG. 15, an increase in the volume of a cell results in a decrease in the volume of its adjacent cell since the driving part 74 acts as a driving component for the two cells 73. When, for instance, the piezoelectric actuator 71 is used as the ink jet head 70 as shown in FIG. 7, the ink cannot be ejected simultaneously from adjacent cells. Therefore, at least two driving operations are needed in order to spray ink particles to an article to be sprayed. This is not preferable from the viewpoint of the improvement in the ink discharging rate.